lecture note radon in geology

Radon in geology

Assistant ProfessorDr. Kamal K. Ali

University of Baghdad-College of Science – Geology Dept.

The geology of radonWhat is Radon?Radon is a gas produced by the radioactive decay of the element radium within the decay of U-238 series. Half life = 3.825 d.When solid radium decays to form radon gas, it loses two protons and two neutrons. These two protons and two neutrons are called an alpha particle, which is a type of radiation. Radon itself is radioactive because it also decays, losing an alpha particle and forming the element polonium.

Radon

• radon (222Rn): One of the main members of natural radioactivity of the earth crust.

• It is a noble gas and having a relatively long life-time (3.8) days.

• It has a great mobility to reach considerable distances in different geological environments.

• Radon is present everywhere: air, soil, water

Radon

• The concentrations of radon in geological environments depend mainly on the migration processes, and the abundance of it parent nucleus Radium (Ra-226)

• Upward migration of radon gas in soil is facilitated by the presence of faults.

• It reaches in the atmosphere by diffusion to the surface, this exhalation forming the radon flux of the earth crust

Radon

• Radon and radon flux from soil are used as indicators for some applications such as:

radon risk assessment by the determination of radon potential of the soil.

identification of the faults.in applying migration models in soil and

geological environmentsand for the transport to the atmosphere and

inside homes

Radon

• There are at least three different issues of great importance in radon studies:

The first issue relates to the presence of radon and radium in ground waters (wells, mineral springs, geothermal waters, etc)

(In addition to the knowledge of radiation doses received by population in using these water sources (by ingestion, inhalation, spa treatment) , radon monitoring in groundwaters and geothermal waters is a great interest in geophysical studies.)

Radon


The second aspect is related to the radon potential in soil and the flux from the earth (soil) surface.

By this, is important that radon anomalies indicate radioactive accumulation (U, Th) or the presence of tectonic faults. In such areas, radon flux from soil is significantly higher

Radon


• The third aspect, is related to radon concentrations inside homes.

• Outdoor air has an average radon concentration of 4-8 Bq m-3 that depends on the geological and ⋅meteorological conditions.

• Inside homes, the radon concentration may produce normal amounts of 20-80 Bq m-3, through ⋅accumulation.

Radon

• In case of high indoor radon levels the main radon sources are the soil and building materials, which contain radioactive materials or uranium waste in uranium areas. These zones are considered “radon-prone areas”

Radon

• In addition to these important aspects of radon studies, another research field are the

applications in geophysics• Radon considered as „trace element” or

„monitoring element”, can give information about geophysical properties of geological formations

Radon

• Another important application: • is the use of radon monitoring techniques in

the studies of volcanic eruptions• and of seismic activities, where the monitoring

radon concentration variations in bore-holes and groundwater can be applied to earthquakes forecast

methods of radon concentration measurement in soil

There are two basic method for measuring radon in soil. Both methods measure the alpha particles produced by the decay of the radon in the air.In the first method, a sample of soil air is collected from a probe driven into the ground, and the radon in the sample is measured by using electronic equipment.

The Rn measuring start after delay time 5 minutes. The total time of one measurement is no more than 10 minutes.the detector determines an average Rn concentration (corrected from the background of the of the cell

Soil-air radon measurement

Solid state nuclear track detector• In the other method involves putting an alpha-

track detector, in the soil and leaving it open to the soil air. This method allows long-term measurements,. It is called Solid state nuclear track detector

• A solid state nuclear track detector (SSNT) is a sample of a solid material ( glass or plastic) exposed to nuclear radiation ( charged particles), etched, and examined microscopically.

Solid state nuclear track detector• The basis of solid state nuclear track detection is that

charged particles damage the detector within nanometers along the track in such a way that the track can be etched many times faster than the undamaged material. Etching, typically for several hours, enlarges the damage to conical pits of micrometer dimensions, that can be observed with a microscope.

• If the particles enter the surface at normal incidence, the pits are circular; otherwise the ellipticity and orientation of the elliptical pit mouth indicate the direction of incidence.

SSNTD

• SSNTDs are commonly used to study radon concentration in houses, and the age of geological samples.

• A material commonly used in SSNTDS is CR39. It is a clear, colorless, rigid plastic with the chemical formula C12H18O7. Etching is usually performed in solutions of caustic alkalis such as sodium hydroxide, often at elevated temperatures for several hours.

SAMPLE OF SSNTD

Calibration Factor

Applications of radon studies in environmental science, geology and geophysics

Radon studies with applications in risk assessment of radon from soil: the assessment of radon risk from soil.

• radon is the main source of natural radiation for the population, with a contribution of about 57 % to annual dose.

• In some areas, this can reach towards to the annual mean exposure of 2.2 mSv. Depending on geological and meteorological conditions,


• In radon risk areas (”radon-prone areas”) the concentration of radon gas in atmosphere and indoor can reach high levels, which is due to soil and building materials.

• The assessment of the radon risk from soil is based on the determination of the radon potential by measuring radon concentration from soil and the permeability of soil.


• models are used for risk assessment of soil radon.

The evaluation of radon index (radon risk)


• The soil gas radon concentration is one of the main parameter in determining the

• radon potential of a building site. Soil permeability is the second main parameter in determining the radon potential of a building site.


• Soil permeability is the second main parameter in determining the radon potential of a building site.

• High permeability allows an increased transport of radon from soil and transfer to the building, thus in case of permeable soils can be estimated an increase radon risk.

• Soil permeability can be determined by in situ measurements, where the k permeability is given in [m2].

• In situ soil permeability measurements usually is carried out at a depth of 0.8 m in soil, and the measurement method consist in measuring the flow rate of the soil gas by extraction or by pumping in soil at constant pressure.


• According to radon risk assessment, the categories of soil permeability are the follows:

• k < 4,0 10-13 m2 for low permeability; ⋅• 4,0 10-13 m2 < k < 4,0 10-12 m2 for medium ⋅ ⋅

permeability;• and k > 4,0 10-12 m2 for high permeability. ⋅• The number of in situ permeability measurements are the

same as for soil radon measurements, at least 15 measurements for a building site (≤ 800 m2 area), or perform measurements in grids of 10x10 m for sites with area > 800 m2.

The 10 point system to estimate the soil radon potential

Applications of radon studies geology and geophysics

Applications of radon studies geology and geophysics

• radon has a role of trace elements, which can indicate accumulation of radioactive material in the crust, or the presence of tectonic faults.

• Faults serving as pathways for the ascendant migration of these gases towards surface.

• Detection of high thoron activities in soil gas may indicate a fast migration mechanism from a distant source, due to the short half-time of thoron (55 sec) than of radon (3,82 days). This is possible only in presence of a carrier gas (e.g., CO2) as typically occurs along faults and fractured rocks

Radon Monitoring for geological exploration

The mobilization of uranium ore body detected by radon measurements using SSNTDs

Use of radon monitoring in hydrocarbon exploration

Sketch of gas seepage from a simplified hydrocarbon Reservoir. The gas emission at the surface of the earth is indicated in the graph. “Reservoir” refers solely to hydrocarbons

Application of radon monitoring in locating geological faults

Radon Hydrologic Studies

Because of its rapid loss to air and comparatively rapid decay, radon is used in hydrologic research that studies the interaction between ground water, streams and rivers. Any significant concentration of radon in a stream or river is a good indicator that there are local inputs of ground water

Radon emission as precursor of earthquake

•The strain changes occurring within the earth's surface during an earthquake is expected to enhance the radon concentration in soil gas. •The principle seems to be simple: Radon gas which is trapped within the ground, is released through small fractures resulting from many changes taking place in the earth's crust in that region prior to the major physical shock of an earthquake..

Radon emission as precursor of earthquake

The stress–strain developed within earth's crust before an earthquake leads to changes in gas transportation and rise of volatiles from the deep earth to the surface. As a result, remarkable quantities of radon come out of the pores and fractures of the rocks on surface

Concentration of radon• Because the level of radioactivity is directly related

to the number and type of radioactive atoms present, radon and all other radioactive atoms are measured in picocuries. For instance, a house having 4 picocuries of radon per liter of air (4 pCi/L) has about 8 or 9 atoms of radon decaying every minute in every liter of air inside the house. A 1,000-square-foot house with 4 pCi/L of radon has nearly 2 million radon atoms decaying in it every minute.

Concentration of radon• Radon levels in outdoor air, indoor air, soil air, and

ground water can be very different. Outdoor air ranges from less than 0.1 pCi/L to about 30 pCi/L, but it probably averages about 0.2 pCi/L. Radon in indoor air ranges from less that 1 pCi/l to about 3,000 pCi/L, but it probably averages between 1 and 2 pCi/L. Radon in soil air (the air that occupies the pores in soil) ranges from 20 or 30 pCi/L to more than 100,000 pCi/L

• The amount of radon dissolved in ground water ranges from about 100 to nearly 3 million pCi/L.

Uranium: The source

To understand the geology of radon - where it forms, how it forms, how it moves - we have to start with its ultimate source, uranium. All rocks contain some uranium, although most contain just a small amount - between 1 and 3 parts per million (ppm) of uranium. In general, the uranium content of a soil will be about the same as the uranium content of the rock from which the soil was derived.

The bright-yellow mineral tyuyamunite is one of the most common uranium ore minerals, Pitchbend UO2, Uraninite, Granotite

Some types of rocks have higher than average uranium contents. These include light-colored volcanic rocks, granites, dark shales, sedimentary rocks that contain phosphate, and metamorphic rocks derived from these rocks. These rocks and their soils may contain as much as 100 ppm uranium.

U-238 decay series

Radon formation

• Just as uranium is present in all rocks and soils, so are radon and radium because they are daughter products formed by the radioactive decay of uranium.

• Each atom of radium decays by ejecting from its nucleus an alpha particle composed of two neutrons and two protons

Radon formation

• The location of the radium atom in the mineral grain (how close it is to the surface of the grain) and the direction of the recoil of the radon atom (whether it is toward the surface or the interior of the grain) determine whether or not the newly formed radon atom enters the pore space between mineral grains.

• If a radium atom is deep within a big grain, then regardless of the direction of recoil, it will not free the radon from the grain, and the radon atom will remain embedded in the mineral.

Radon formation

• Even when a radium atom is near the surface of a grain, the recoil will send the radon atom deeper into the mineral if the direction of recoil is toward the grain's core. However, the recoil of some radon atoms near the surface of a grain is directed toward the grain's surface. When this happens, the newly formed radon leaves the mineral and enters the pore space between the grains or the fractures in the rocks.

The recoil of the radon atom is quite strong. Often newly formed radon atoms enter the pore space, cross all the way through the pore space, and become embedded in nearby mineral grains. If water is present in the pore space, however, the moving radon atom slows very quickly and is more likely to stay in the pore space. For most soils, only 10 to 50 percent of the radon produced actually escapes from the mineral grains and enters the pores

Radon movement

• Because radon is a gas, it has much greater mobility than uranium and radium, which are fixed in the solid matter in rocks and soils. Radon can more easily leave the rocks and soils by escaping into fractures and openings in rocks and into the pore spaces between grains of soil.

• The ease and efficiency with which radon moves in the pore space or fracture effects how much radon enters a house. If radon is able to move easily in the pore space, then it can travel a great distance before it decays, and it is more likely to collect in high concentrations inside a building.

Radon movement

• The method and speed of radon's movement through soils is controlled by the amount of water present in the pore space (the soil moisture content), the percentage of pore space in the soil (the porosity), and the "interconnectedness" of the pore spaces that determines the soil's ability to transmit water and air (called soil permeability).

Radon moves more rapidly through permeable soils, such as coarse sand and gravel, than through impermeable soils, such as clays. Fractures in any soil or rock allow radon to move more quickly.

• Radon in water moves slower than radon in air. The distance that radon moves before most of it decays is less than 1 inch in water-saturated rocks or soils, but it can be more than 6 feet, and sometimes tens of feet, through dry rocks or soils. Because water also tends to flow much more slowly through soil pores and rock fractures than does air, radon travels shorter distances in wet soils than in dry soils before it decays.

lecture note radon in geology

Education

Transcript of lecture note radon in geology